Lapidary grinding system



Jan. 23, 1968 H. M. ZALEWSKI LAPIDARY GRINDING SYSTEM Filed 061,. 23, 1965 HENRY M. ZALEWSKI INVENTOR.

ATTO RNEY United States Patent 3,364,629 LAPIDARY GRINDING SYSTEM Henry M. Zalewski, 194 Washington Ave, Carteret, NJ. 070% Filed Get. 23, B65, Ser. No. 593,165 6 Claims. (Cl. 51-433) This invention relates to the grinding of gem stones, semiprecious stones and the like. In particular, this invention relates to methods for the grinding of convex curved surfaces in the nature of shallow cabochon like cuts.

A popular type of jewelry makes use of rather wide, flat stones such as turquoise, agate and the like, frequently of oval outline, which are mounted in appropriate settings for use as brooches, pendants, bracelets and the like. Such stones are commonly ground to a shallow convex curvature, so that the central portion of the stone is higher than the marginal portions adjacent the setting, as such grinding has been found to greatly enhance the attractiveness of the article, bringing out the natural beauty of the stone.

Stones of this type are usually mounted in a rim setting which embraces the entire periphery of the stone, rather than a prong setting of the type used, for example for diamonds and the like. Rim settings are .more suitable for stones such as agate, turquoise, etc. for a variety of reasons. For one thing, the rim setting protects the edges of the stone, and stones such as agate, turquoise and the like, lacking the extreme hardness of diamond, corundum, emerald and the like, have greater need of such protection. Moreover, the rim type of setting is inherently more rugged and therefore more desirable. it is not used in the mounting of transparent stones such as diamonds, because of the necessity of providing for the greatest possible access of light to the interior of the stone. Stones such as agate, turquoise, etc., on the other hand are for the most part opaque or only slightly translucent, and do not depend upon internal refraction and reflection to enhance their beauty, so that the inherently more rugged rim setting can be used without loss of attractiveness of the stone.

In order to employ the rim setting, however, it is important to provide a stone having a girdle of substantially uniform height all around the periphery of the stone. This is necessary because the rim setting, as manufactured, has an upstanding flange or rim which is of uniform height, which is pressed down around the edge of the stone to hold it in the setting. The turned down edge of the rim should follow closely the contour of the edge of the stone, overlapping the face of the stone by a small, uniform amount all the way around the periphery. When the height of the girdle varies from point to point, however, this is impossible, because the overlapping portion of the rim will be larger where the girdle height, and smaller, or may even miss the edge of the stone entirely, where the girdle is high. This disadvantage may be compensated for, for example by trimming the rim to correspond with the varying height of the girdle, but only at the cost of considerable tedious effort and expense.

The g inding of stones of the type under consideration is ordinarily done by manual methods, whereby the stone, mounted on a suitable dop or the like, is manually held in contact with an abrasive wheel and moved about in such a Way as to develop the desired curvature, guided primarily by the eye of the lapidary. While developing the desired curvature, the lapidary also attempts to do so in such a way that the girdle will be left with a uniform height all around. Grinding the stone manually to satisfy both requirements (facial curvature and uniform girdle is of lesser 3,354,629 Patented Jan. 23, 1968 height) requires a great deal of skill on the part of the lapidary, and the difficulties are aggravated when, as frequently happens, the hardness of the stone is not uniform throughout.

Mechanical grinding methods, whereby the precision of a machine could be at least partially substituted for the mechanical skill of the operator, have thus far proved unsatisfactory, because they are designed primarily to produce spherical curves and a spherical curve on the surface of an oval stone produces a girdle of non-uniform height, being high adjacent the ends of the short axis of the face, and low adjacent the ends of the long axis thereof.

An object of the present invention, therefore, is to provide an improved lapidary method.

Another object is to provide an improved method for grinding shallow curves in the face of fiat stones.

A further object is to provide a method as aforesaid, which is adapted to leave a girdle of substantially uniform height around the periphery of the stone.

Still another object is to provide a method for grinding an aspherical curve on a face of a flat stone.

Yet anbther object is to provide a method of the type described, which can readily be adapted for mechanical grinding.

A feature of the invention is the use of a grinding tool having a working face characterized by a compound curvature.

Another feature is the use of a grinding tool having a concave working face with a curvature referrable to two mutually perpendicular axes, the radius of curvature along one of said axes being substantially difierent from the radius of curvature along the other of said axes.

Still another feature is the orientation of an oval stone with respect to the grindin tool, in such a Way that the major and minor axes of the oval stone coincide substantially with the axes of curvature of the grinding tool.

A further feature is the grinding of the stone by a primarily reciprocating motion in contact with the grinding tool.

According to another feature of the invention, the radii of curvature along the axes of curvature of the grinding tool are related to the lengths of major and minor axes of the stone substantially according to a mathematical relationship hereinafter more fully set forth.

Other objects, features and advantages will become apparent from the following more complete description and claims, and from the accompanying drawings.

In one particularly desirable embodiment, this invention contemplates a method for grinding a decorative stone which comprises the steps of providin a generally flat blank of said stone having a periphery bounded by a girdle and having a plan outline of oval configuration having substantially mutually perpendicular major and minor diameters, providing a grinding tool having a concave working face characterized by a pair of substantially mutually perpendcular axes and having a major radius of curvature along one of said axes and a minor radius of curvature along the other of said axes, orienting said blank in said grinding tool with said major diameter of said stone generally parallel with the axis of said grinding tool characterized by said major radius of curvature, and grinding said blank by predominantly translational motion of said blank in contact with said concave working face of said grinding tool, While substantially maintaining the orientation of the axes of said blank with the axes of said tool.

Referring now to the drawings:

FIGURE 1 is a simplified, somewhat diagrammatic perspective representation of a grinding tool and a blank in mutually oriented condition for the practice of the present invention.

FIGURE 2 is a diagram illustrating the geometric relationships between the blank and the grinding tool and illus trating the considerations leading to the development of the guide formula hereinafter set forth.

As illustrated in FIGURE 1, the workpiece to which the method of the present invention is most usually applied is an oval blank of a decorative or semi-precious stone, for example quartz, agate, onyx, jasper, jet, turquoise, or similar material, of generally flat configuration. The oval outline generally approximates an ellipse, but it need not be a mathematically accurate ellipse in order to profit by the advantages of the present invention. In this, as in other aspects of the invention more fully discussed hereinafter, it is to be understood that while mathematical and geometric analysis has been employed to illustrate an idealized theoretical concept of the invention, appreciable departures from mathematical precision can be tolerated, without introducing inaccuracies sufiiciently large to negate the advantages of the grinding method of the invention.

To grind the stone, the blank 10 is placed in a grinding tool 12 having a concave working face 14. The concavity of the working face 14 is aspherical, having a larger radius of curvature along one axis designated XX and a smaller radius of curvature along a second axis YY, which is perpendicular to axis XX. Vertical cross-sections through XX and Y-Y preferably present circular arcs in profile and such will be assumed in the balance of this disclosure. It is not essential that they be circular arcs, however, and elliptical, sub-elliptical, super-elliptical or other curve segments may be used instead if desired.

The blank is oriented in the grinding tool with its longer dimension substantially parallel with the axis of the tool having the larger radius of curvature, i.e. axis XX in FIGURE 1. The grinding tool is charged with an appropriate abrasive such as emery grit, aluminum oxide, etc., and the stone is ground, for example by a reciprocating motion in the direction indicated by the double-headed arrow. The motion of the stone relative to the grinding tool may, if desired, be transverse rather than longitudinal, or may be a combination of transverse and longitudinal motions, producing a rotary or more complex movement. Throughout the grinding, however, the stone should be maintained in the general orientation shown, with the long axis of the stone substantially parallel to the major axis (i.e. the axis characterized by the larger radius of curvature) of the tool. Notwithstanding the foregoing, it is permissable and frequently desirable to accompany the overall translational movement of the stone by a slight rocking or horizontal rotational motion, oscillating about the parallel orientation just described. Such rocking helps to avoid the development of undesirable striations in the surface of the stone. The grinding is continued in this fashion until the stone has been brought to the desired surface contour. Assuming that the aspherieal curvature of the tool employed is reasonably closely related to the relationship between the major and minor diameters of the blank, as more fully described below, the stone will be found to have a girdle of substantially uniform height on completion of the grinding operation.

Following grinding to the desired shape, the stone is polished. This may be done in any convenient or conventional manner, but according to a particularly effective and convenient aspect of the invention, the stone is polished in the same tool in which it was ground, by simply interposing a felt or the like between the grinding tool and the face of the stone, charging the felt with a polishing abrasive such as rouge or cerium oxide, and continuing the same motion as was used for the grinding. The grinding, and also the polishing, may be carried out in stages, using successively finer grits, as will be obvious to those skilled in the art.

The relationship between the dimensions of the blank and the contour of the aspherical working face of the tool may be best understood by reference to FIGURE 2. FIG- URE 2 is a diagrammatic composite view showing, in

superimposed relationship, a cross-section of the blank 10 taken through its minor diameter in contact with the minor axis of the tool, and a cross-section of the blank taken through its major diameter in contact with the major axis of the tool. The cross-section of the stone through its minor diameter is bounded by points A, C, D, E, F and the crosssection through the major diameter is bounded by points A, CD, E, F. The profile of the grinding tool through its minor axis is the arc GG and the cross-section of the tool through its major axis is the arc HH. 0 is the origin or center of the circular arc GG and O is the origin or center of arc HH. The point B represents the intersection of line OO'C, through the center of the stone, with the line AADD, drawn perpendicular to line OO'C through the points at which the girdle intersects the ground face of the stone. R represents the radius of curvature of the arc HH, and r the radius of curvature of the arc GG. S designates the major semidiameter of the blank (or of the ground stone) and s the minor semidiameter thereof. It will be noted that the distances A F, AF, DE and D E are all equali.e. the girdle is of the same height at all points. The maximum height of the crown above the girdle, i.e. the distance BC, is designated h.

It will be noted that FIGURE 2 defines two right triangles, namely triangle A=BO and triangle A'BO. Considering the smaller triangle ABO, the leg AB thereof is the minor semidiameter s of the blank and the hypotenuse A0 is the minor radius of curvature r of the grinding tool. The leg OB may therefore be represented by the quantity ,a 2 Since the distance 0C also equals the minor radius of curvature r of the tool, it is apparent that the distance BC, i.e. the height It, may be represented by the equation curvature R of the grinding tool. The vertical leg O' B can therefore be represented by the quantity /R S Since the distance OC also equals the major radius of curvature R, it is evident that the distance BC, or h, satisfies the equation h=R /R S (2) Combining equations (1) and (2):

R- /R S =r /r s (3) Equation 3 expresses the relationship between the dimensions of the major and minor semi-diameters of the stone and the major and minor radii of curvature f the tool face theoretically necessary to give a mathematically precise equality between the height of the girdle at the ends of the stone and the height of the girdle at the sides.

In order to determine the proper radii to be used for grinding a blank of given dimensions, it is most convenient to use equations derived as follows:

Referring to the smaller triangle ABO, it is apparent as shown above that The value of h is a matter of choice, within the limitations of the blank thickness, and depends on whether a nearly fiat crown is desired or a higher, more rounded crown.

Equation 2 can be rearranged in the same manner to give the equation Assuming, by way of example, that it is desired to grind a blank having a major semidiameter S of 43 mm. and a minor semidiameter of 34 mm. to a cabochon having a height h of above the girdle, the radii are determined as follows:

By equation (5):

k R 2X 0 7.5 mm. By equation (4):

r =62.8 mm.

An appropriate grinding tool is then selected or prepared, having a radius of curvature of 97.5 mm. or a reasonable approximation thereto, along its major axis (X-X in FIGURE 1) and a radius of curvature reasonably approximating 62.8 mm. along its minor axis YY in FIG- URE 1). The blank is then oriented in the tool and ground in the manner already described.

The degree of precision required in matching the actual radii of curvature of the tool to the radii determined by the above equations depends in part on the precision required for the particular work at hand, but is always subject to some degree of tolerance. In the above example, if a tool is chosen with radii of 100 and 65 mm., instead of 97.5 and 62.8 mm., respectively, the girdle will still be of uniform height (always assuming that the table, or rear of the blank is flat to begin with), within a tolerance of approximately 0.1 mm. Where larger variations are permissible, larger deparatures from theoretical may be taken. Also, the cutting selected for the above example is a relatively steep one, and where flatter cuttings are being made, proportionally greater variations from the theoretical radii of the tool are permissible.

To save time and unnecessary wear on the aspherical grinding tool, it is frequently desirable to perform a preliminary grinding by manual or by other mechanical means, thereby removing most of the material which is to be ground away, so that the blank when placed in the aspherical grinding tool may have a spherically or cylindrically curved face, or even an irregular curvature. Such preliminary grinding does not materially affect the results achieved by the final grinding and polishing operations. While this invention has been described in terms of certain preferred embodiments and illustrated by way of certain examples and drawings, these are illustrative only, as many alternatives and equivalents will readily occur to those skilled in the art, without departing from the spirit or proper scope of the invention. The invention is therefore not to be constued as limited, except as set forth in the appended claims.

What is claimed is:

1. A method for grinding a decorative stone, comprising in combination the steps of providing a generally flat blank of said stone having a periphery bounded by a girdle and having a plan outline of oval configuration having substantially mutually perpendicular major and minor diameters, providing a grinding tool having a concave working face characterized by a pair of substantially mutually perpendicular axes and having a major radius of curvature along one of said axes and a minor radius of curvature along the other of said axes, orienting said blank in said grinding tool with said major diameter of said stone generally parallel with the axis of said grinding tool characterized by said major radius of curvature, and grinding said blank by predominantly translational motion of said blank in contact with said concave working face of said grinding tool, while substantially maintaining the orientation of the axes of said blank with the axes of said tool.

2. A method according to claim 1, wherein said predominantly translational motion is accompanied by a rocking motion of minor amplitude compared to the amplitude of said translational motion, said rocking motion being a horizontal rotational motion of said blank in contact with said tool.

3. A method according to claim 1, wherein said grinding tool is characterized by an aspherieally-curved concave working face having a contour generally corresponding to the desired contour of said blank after grinding thereof.

4. A method according to claim 1, wherein said blank is polished after grinding thereor" to the desired contour, by interposing a resilient cushion between said grinding tool and said blank, charging said cushion with a polishing abrasive, and subjecting said blank to predominantly translational motion in contact with said cushion and said polishing abrasive.

5. A method according to claim 1, wherein the major and minor semidiameters of said blank and the major and minor radii of curvature of said blank are such as to approximately satisfy the equation:

wherein R represents the major radius of curvature of the grinding tool, r represents the minor radius of curvature thereof, S represents the major semidiameter of the blank, and s represents the minor semidiameter thereof.

6. A method according to claim 1, wherein said blank is preliminarily ground to remove a major portion of the material to be ground away, prior to placing said blank in said grinding tool.

References Cited UNITED STATES PATENTS 369,431 9/ 1887 Brown 51-284 X 445,349 1/1891 Bausch 51-60 X 2,168,843 8/1939 Lockhart 516O X LESTER M. SWINGLE, Primary Examiner. 

1. A METHOD FOR GRINDING A DECORATIVE STONE, COMPRISING IN COMBINATION THE STEPS OF PROVIDING A GENERALLY FLAT BLANK OF SAID STONE HAVING A PERIPHERY BOUNDED BY A GIRDLE AND HAVING A PLAN OUTLINE OF OVAL CONFIGURATION HAVING SUBSTANTIALLY MUTUALLY PERPENDICULAR MAJOR AND MINOR DIAMETERS, PROVIDING A GRINDING TOOL HAVING A CONCAVE WORKING FACE CHARACTERIZED BY A PAIR OF SUBSTANTIALLY MUTUALLY PERPENDICULAR AXES AND HAVING A MAJOR RADIUS OF CURVATURE ALONG ONE OF SAID AXES AND A MINOR RADIUS OF CURVATURE ALONG THE OTHER OF SAID AXES, ORIENTING SAID BLANK IN SAID GRINDING TOOL WITH SAID MAJOR DIAMETER OF SAID STONE GENERALLY PARALLEL WITH THE AXIS OF SAID GRINDING TOOL CHARACTERIZED BY SAID MAJOR RADIUS OF CURVATURE, AND GRINDING SAID BLANK BY PREDOMINANTLY TRANSLATIONAL MOTION OF SAID BLANK IN CONTACT WITH SAID CONCAVE WORKING FACE OF SAID GRINDING TOOL, WHILE SUBSTANTIALLY MAINTAINING THE ORIENTATION OF THE AXES OF SAID BLANK WITH THE AXES OF SAID TOOL. 